A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
EUV optical systems like lithographic apparatus may include both normal incidence reflectors (plane or curved mirrors) and grazing incidence reflectors. There is no material that by itself forms a good reflector of EUV radiation, so that the normal incidence mirrors need to be constructed as a multilayer structure with tens of pairs of alternating layers of different materials, whose composition and thicknesses are tuned precisely to the wavelength of the radiation to be reflected. The layer thicknesses (hence the period of the multilayer repeating structure) must also be set with regard to the angle of incidence of the radiation. Grazing incidence reflectors, on the other hand, have a much simpler structure, typically a single mirror surface layer of, for example, ruthenium (Ru). Reflectivities at EUV wavelengths in both types of reflector are already low compared to reflectors at longer wavelengths, which is a particular problem since a typical EUV lithographic system may have several mirrors. For example, a EUV lithographic system may have nine mirrors: two in the illumination optics, six in the imaging optics plus the reflecting mask. It is therefore evident that even a small decrease of 1% in the peak reflectance of a single mirror will cause a significant light throughput reduction in the optical system.
The reflectance of a grazing incidence reflector decreases markedly as the angle of incidence rises toward normal. The wavelength of radiation in a present lithographic apparatus is about 13.5 nm and a Ru layer reflects 50% of the incident radiation only up to around 25 degrees incidence angle (where zero degrees means parallel to the reflector surface). To exploit higher numerical aperture sources, it would be valuable to be able to increase this angle, even by a few degrees. For future systems with a shorter wavelength of radiation, for example about 6.7 nm, it is found that the reflectance of a grazing incidence mirror decreases even more rapidly with the increase of grazing incidence angles. Again, the ability to increase the acceptance angle by even a few degrees would contribute greatly to the efficiency and performance achievable in such systems.